Radio Frames in Long Term Evolution (LTE) systems include frame structures in a Frequency Division Duplex (FDD) mode and a Time Division Duplex (TDD) mode. In a frame structure in the FDD mode, as shown in FIG. 1, a radio frame of 10 milliseconds (ms) consists of 20 slots, numbered from 0 to 19, each having a length of 0.5 ms. The slot 2i and the slot 2i+1 form a subframe i having a length of 1 ms. In a frame structure in the TDD mode, as shown in FIG. 2, a radio frame of 10 ms consists of two half frames each including five subframes each having a length of 1 ms. A subframe i is defined as two slots 2i and 2i+1, each having a length of 0.5 ms.
In the above two frame structures, for a normal Cyclic Prefix (CP), a slot contains 7 symbols each having a length of 66.7 microseconds (μs), with the first symbol having a CP length of 5.21 μs and the remaining six symbols each having a CP length of 4.69 μs. For an extended CP, a slot contains 6 slots each having a CP length of 16.67 μs. A time unit Ts is defined as Ts=1/(15000×2048) seconds. The supported uplink-downlink configurations are shown in Table 1 below. For each subframe in a radio frame, “D” represents a subframe dedicated for downlink transmission, “U” represents a subframe dedicated for uplink transmission, and “S” represents a special subframe for three fields: a Downlink Pilot Time Slot (DwPTS), an Uplink Pilot Time Slot (UpPTS) and a Guard Period (GP). The lengths of DwPTS and UpPTS are shown in Table 2, subject to a constraint that the total length of DwPTS, GP and UpPTS is 30720·Ts=1 ms. Each subframe i is represented by two slots 2i and 2i+1. Each slot has a length of Tslot=15360·T=0.5 ms.
The LTE TDD supports an uplink-downlink switching period of 5 ms or 10 ms. If the period of the downlink-to-uplink switching point is 5 ms, the special subframe will occur in each of the two half frames. If the period of the downlink-to-uplink switching point is 5 ms, the special subframe will occur in the first half frame only. Subframe 0, Subframe 5 and the DwPTS are always used for downlink transmission, while the UpPTS and the subframe immediately following the special subframe are dedicated for uplink transmission.
TABLE 1Uplink-Downlink ConfigurationDownlink-UplinkUplink-Switching DownlinkPointSubframe No.ConfigurationsPeriod01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD6 5 msDSUUUDSUUD
TABLE 2Special Subframe Configuration (DwPTS/GP/UpPTS Length)Normal CP, DownlinkExtended CP, DownlinkSpecialUpPTSUpPTSSubframeNormal CP,ExtendedNormal CP,Extended CP,ConfigurationDwPTSUplinkCP, UplinkDwPTSUplinkUplink0 6592 · Ts2192 · Ts2560 · Ts 7680 · Ts2192 · Ts2560 · Ts119760 · Ts20480 · Ts221952 · Ts23040 · Ts324144 · Ts25600 · Ts426336 · Ts 7680 · Ts4384 · Ts5120 · Ts5 6592 · Ts4384 · Ts5120 · Ts20480 · Ts619760 · Ts23040 · Ts721952 · Ts———824144 · Ts———
In the LTE, a Physical Downlink Control Channel (PDCCH) carries uplink and downlink scheduling information as well as uplink power control information. There are various Downlink Control Information (DCI) format: 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 3, 3A, etc. Abase station, also known as evolved NodeB (eNB), can configure a terminal device, also known as User Equipment (UE), via DCI. Alternatively, a terminal device can receive a higher layer configuration. In other words, a UE can be configured via higher layer signaling.
Broadcast information in the LTE system includes Master Information Block (MIB) and System Information Block (SIB). The MIB is transmitted on Physical Broadcast Channel (PBCH) and the SIB, also known as Scheduled Information (SI), is transmitted on Physical Downlink Shared Channel (PDSCH). Depending on scheduling requirements (e.g., periods), the SIB information can be divided into several pieces of SI for transmission. At least four pieces of SI are supported, i.e., SIB-1, SIB-2, SIB-3 and SIB-4.
Sounding Reference Signal (SRS) is a signal used for measuring wireless Channel State Information (CSI) between the terminal device and the base station. In the LTE system, the UE transmits an uplink SRS at the last data symbol in a transmission subframe in accordance with parameters indicated by the eNB, such as a bandwidth, a frequency position, a sequence cyclic shift, a period and a subframe offset. The eNB determines the uplink CSI of the UE based on the received SRS and performs operations, such as frequency-domain selection and scheduling, closed-loop power control and the like, based on the determined CSI.
Within one single SRS bandwidth, a number of UEs may transmit SRSs in one single frequency bin with different cyclic shifts by means of code division multiplexing, or two UEs can transmit SRSs in different frequency bins by means of frequency division multiplexing. For example, in the LTE system, a UE can use one of eight cyclic shifts and one of two frequency bins for transmitting an SRS within a particular SRS bandwidth (i.e., four Resource Blocks (RBs)). That is, the UE has in total 16 resources available for SRS transmission. In other word, up to 16 SRSs can be transmitted simultaneously within the SRS bandwidth. Since the LTE system does not support Single User-Multiple Input Multiple Output (SU-MIMO) in uplink, the UE can only transmit an SRS via one antenna at a time. Hence, one UE only needs one SRS. Accordingly, up to 16 UEs can be multiplexed within the above SRS bandwidth.
The LTE-Advanced (LTE-A) system is the next generation of the LTE system. It supports SU-MIMO in uplink and can use up to four antennas for uplink transmission. That is, a UE can transmit an SRS via more than one antenna simultaneously and an eNB needs to estimate the state of each channel based on the SRS received at each antenna.
In a related LTE-A research, it has been proposed to use a non-precoded (i.e., antenna specific) SRS in uplink communications. In this case, when a UE transmits a non-precoded SRS using multiple antennas, the SRS resources required for each UE will increase, resulting in a reduced number of UEs that can be multiplexed simultaneously in the system. Further, in addition to the periodic SRS transmission in the LTE, it is possible to configure a UE to transmit SRSs aperiodically via DCI or higher layer signaling.
For example, within a particular SRS bandwidth (i.e., 4 RBs), if each UE transmits an SRS via four antennas, it will require 4 resources. Recall that 16 SRS resources in total can be supported in one SRS bandwidth, the number of UEs that can be multiplexed within this SRS bandwidth is reduced to 4. Accordingly, the number of UEs that can be multiplexed simultaneously in the system is reduced to ¼ of that in the LTE system. In a related LTE-A Release 10 research, it has been proposed that a UE can be triggered to transmit an SRS via higher layer signaling (also referred to as trigger type 0) or DCI (also referred to as trigger type 1). The SRS triggered via higher laying signaling is a periodic SRS and the SRS triggered via DCI is an aperiodic SRS. In the LTE-A Release 10, the aperiodic SRS transmission is introduced, which improves utilization of the SRS resources and flexibility in resource scheduling to some extent.
In a future LTE-A Release 13 research, in a scenario of Full Dimension-MIMO (FD-MIMO) or Massive-MIMO, with the increase in demands for SRS measurements by TDD channel reciprocity and the increase in the number of UEs to be multiplexed, it is difficult for the current multiplexing capacity of SRSs to fulfill those requirements.